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Degradation of coral reef ecosystems are recognized in many areas around the world. Increase of seawater turbidity due to terrigenous sediments is regarded as one of the major cause of the degradation. We investigated levels and temporal variability of the turbidity during a two-year period at Urasoko Bay, a fringing coral reef of Ishigaki Island, Japan. At the near shore station, the median value of the turbidity was 2.26 NTU with the maximum of 92.9 NTU. At the two stations, 150m and 370m offshore, the median values of the turbidity were 0.58 NTU and 0.36 NTU, respectively. Correlation of the turbidity with the precipitation was weak, but wind showed significant correlation with the turbidity. Especially, the wind of the northwestern direction tended to increase the turbidity. These observations suggested that re-suspension of the bottom sediments by the wind driven turbulence and the wind-current with high turbidity and low salinity affected the variability of the turbidity in the Urasoko Bay.

The distributions of sediments and its relation to reef water turbidities were investigated in three fringing reefs along the east coast of Ishigaki Island and a patch reef named “Shimobishi” in the lagoon of Sekisei Reef. In order to quantify fine particles in sediments, we employed “SPSS (content of suspended particles in sea sediments)” method, which was originally proposed by Ohmija (1987) for quick assessment of land-derived soil distribution in reef sediments. We found highest SPSS values in a moat and channel bottom of the Miyara Bay and along the northern coast of the Todoroki River mouth in Ishigaki Island. In contrast, SPSS values were low in other three reefs including Yasurazaki, Shiraho and Shimobishi areas. Reef water turbidities showed significant correlation with SPSS values, which suggests that the major cause of reef water turbidities is resuspension of fine particles in the sediments. Accuracy and precision of the measurements can be increased by using a portable turbidimeter in the original SPSS method. The modified method has the potential for applying to a study of under-water light environment, which may influence coral-algal competitions in degrading reef conditions.